Page 57 - Introduction to chemical reaction engineering and kinetics

2.7 Stoichiometric Table 39

column are given in the next three columns for a BR, CSTR, and PFR in turn. The
equation number in the text is given in each case. The results for items (1) (2) and (3)
in the first column apply to either variable or constant density. Those under item (4)
apply only to the special case of a constant-density system.

2.7 STOICHIOMETRIC TABLE
A useful tool for dealing with reaction stoichiometry in chemical kinetics is a “stoichio-
metric table.” This is a spreadsheet device to account for changes in the amounts of
species reacted for a basis amount of a closed system. It is also a systematic method of
expressing the moles, or molar concentrations, or (in some cases) partial pressures of
reactants and products, for a given reaction (or set of reactions) at any time or position,
in terms of initial concentrations and fractional conversion. Its use is illustrated for a
simple system in the following example.

For the gas-phase oxidation of ethylene to ethylene oxide, construct a stoichiometric table
in terms of moles on the basis that only the reactants are present initially, and ethylene is
the limiting reactant.

SOLUTION
The stoichiometry of the reaction is represented by the equation

1
C,H,W + 2W) = VW(g)

A stoichiometric table is constructed as follows:

species ( initial state ( change ( final state

As indicated, it is suggested that the table be constructed in symbolic form first, and nu-
merical values substituted afterwards. If molar amounts are used, as in the table above,
the results are valid whether the density is constant or not. If density is constant, molar
concentrations, ci, may be used in a similar manner. If both density and temperature are
constant, partial pressure, pi, may be used in a similar manner.
The first column lists all the species involved (including inert species, if present).
The second column lists the basis amount of each substance (in the feed, say); this is
an arbitrary choice. The third column lists the change in the amount of each species
from the basis or initial state to some final state in which the fractional conversion is
fA. Each change is in terms of fA, based on the definition in equation 2.2-3, and takes
the stoichiometry into account. The last column lists the amounts in the final state as the
sum of the second and third columns. The total amount is given at the bottom of each
column.